The specific objectives are to: (1) determine the normal erythrocyte concentrations of the important phosphorylated organic compounds (adenosine triphosphate, 2,3 diphosphoglycerate, and inositol hexaphosphate, if present) in the erythrocytes of representative and selected amphibians and reptiles; (2) determine the degree by which these compounds modify the oxygen dissociation curve and thereby influence delivery of oxygen to tissues; (3) determine the effects of temperature and acid-base balance on the concentration of ATP and DPG in the erythrocytes; (4) determine the effect of muscular activity on the level of these compounds in the erythrocytes of selected species; (5) determine the effect of prolonged submerison (in selected aquatic air-breathing species) on the concentration of these compounds; (6) determine the effect of ambient PO2 on the erythrocytic levels of these compounds by examination of species from high altitudes; (7) determine the ontogenetic change in erythrocyte levels of these compounds during development of metamorphosing amphibians (Rana catesbeiana and Ambystoma tigrinum); (8) examine all of the results obtained for any patterns might elucidate the adaptive changes which have taken place during the evolution of vertebrates.